An abnormal alkaline phosphatase, called phosphatase-N, has been reported to occur in the serum from patients with lymphoproliferative diseases including acute and chronic leukemia, lymphosarcoma and infectious mononucleosis. Phosphatase-N does not hydrolyse cysteamine-S-phosphate. Also infectious mononucleosis serum inhibits alkaline phosphatase activity in normal serum. We propose to investigate the molecular nature of phosphatase-N in comparison to normal isozymes of alkaline phosphatase and to identify the inhibitor of alkaline phosphatase that occurs in serum from people with infectious mononucleosis.